Although mannitol has been shown to reduce myocardial infarct size when administered before or soon after coronary artery occlusion, we found that it had no effect when administered three hours after the onset of ischemia. In contrast, nitroglycerin/methoxamine increased collateral blood flow and reduced ST segment elevation (an index of tissue damage). Thus, mannitol, in contrast to nitroglycerin/methoxamine is unlikely to be effective in reducing infarct size in patients with acute myocardial infarction.